1. Field of the Invention
This invention relates to horological systems comprised of a plurality of slave clocks driven from a master time controller and is more particularly concerned with a method and apparatus for improving the time keeping accuracy of the master time controller.
2. Description of the Prior Art
The above cross referenced master time controller, hereinafter referred to as "system", discloses a clock system which is operated selectively from a 50 Hz. or 60 Hz line current or a piezoelectric crystal oscillator circuit. A frequency, whether generated by line current or from the oscillator, is subdivided to provide sub-multiples of the frequency in the form of spaced timing pulses from which pulse trains are generated to drive respective stepper motors of a plurality of slave clocks. The stepper motors and thus the clocks are intermittently advanced a predetermined discrete amount in response to each pulse coupled thereto. Therefore, stepper motor advancement in any given time period is directly related to the number of timing pulses applied in that time. In normal operation of the system, these timing pulses occur at a rate of 24 Hz. which cause the stepper motors to advance at a rate of one revolution per minute, which in turn is translated to a time indication by the clocks. Any inaccuracy in the timing pulse rate will be directly reflected in the time indication. Over long periods of time, a small constant inaccuracy will accumulate into a large error of time indication. Normally, the inaccuracy of 50 Hz. or 60 Hz. line current a selective time base for timing pulses is averaged out over long periods of time. However, short inaccuracies may be relatively large and unacceptable for certain purposes. The inaccuracies of the piezoelectric crystal oscillator, the other selective time base for the timing pulses, will be very small but will nevertheless accumulate over long periods of time.
Accordingly, the system's circuitry is such that the timing pulse rate can be increased to 32 Hz. or decreased to 16 Hz. for correctional purposes. When the timing pulse rate is 32 Hz., the system is said to be in an advanced mode, causing the clocks to gain one second each three seconds. When the timing pulse rate is 16 Hz., the system is said to be in a retarded mode, causing the clocks to loose one second each three seconds. The advanced mode and the retarded mode can be initiated and stopped manually thereby providing a means of correcting errors in the time indication; however, the overall accuracy of the system will be dependent on the accuracy and frequency of manual adjustment.
As to the concept of time synchronization and automatic time adjustment of a clock, the patent to R. Marti, U.S. Pat. No. 3,530,663 discloses an automatic timing setting device for an electronic clock in which correction signals are obtained by a continuous pulse-by-pulse comparison of the oscillating frequency of the clock or a division thereof with the time "ticks" of a broadcasted reference frequency. If the time ticks for any reason are not received, i.e. sensed, the error will be carried forward.
The applicants are also aware of the following additional United States patents relating generally to the present invention: U.S. Pat. Nos. 2,595,431, 3,128,465, 3,217,258, 3,590,573, 3,648,173, 3,685,278, 3,697,689, 3,748,846, 3,861,134, 3,881,310, 3,920,918, 3,921,076, 3,940,558.